Combustion driven heating apparatus contain certain combinations of burners, combustion chambers heat exchangers and furnace stacks which result in resonance events that produce excessive noise levels. The natural frequency for an air column, as is present in heat exchangers and furnace stacks, is described by v=c/2L, where c is the average speed of sound in the flue gas and L is the total length of the air column. For reasons that are not completely understood, the combustion process will occasionally oscillate in phase with this frequency. This oscillation produces a standing acoustic wave inside of the heat exchanger or furnace stack in the same manner as a standing wave exists in an organ pipe. The standing acoustic wave produces harmonics in the frequency range of 50 Hertz to 300 Hertz. Intense tones in this frequency range are produced and are very difficult to treat without affecting the performance of the heating apparatus.
Combustion driven heating apparatus also produce other types of objectionable noise. One of these types of noise, known as screeching, is relevant to high velocity power burners where the sound produced is caused by frictional flow due to high rates of gas flows within the burner. This type of noise may be reduced, and sometimes eliminated, by treating it at or near the burner itself. For example U.S. Pat. No. 2,154,133 discloses a method of attenuating the noise downstream of the burner with ducting treated with acoustic insulation. In U.S. Pat. No. 3,684,424 there is disclosed a method of enclosing the burner in a housing treated with acoustic insulation to attenuate the screeching type noise. While in U.S. Pat. No. 5,017,129 there is disclosed a method of mixing two streams of air at the burner to reduce the overall frictional effects that produce the screeching noise. While these burners reduce or eliminate the frictional flow form of noise they do little to reduce the resonance form of noise produced by gas fired heating apparatus.
Another type of noise generated by combustion driven heating apparatus is broadband in nature and is a result of fluctuating density in the flame emanating from the burner itself. It is known in the industry to treat this type of noise by suppressing or eliminating through the use of acoustic insulation as disclosed in U.S. Pat. No. 4,029,462. U.S. Pat. No. 4,175,919 discloses a method of attenuating this type of noise by providing a first burner with a laminar flow and a second burner having a turbulent flow. In U.S. Pat. No. 5,344,308 a method is disclosed to treat this type of noise by providing a number of holes in the combustion chamber downstream of the burner to allow combustion type noise to escape from the combustion chamber. These inventions are also focused on reducing or eliminating noise from a specific source and therefor do little to reduce the resonance form of noise produced by a combustion driven heating apparatus. It should also be noted that the reduction or elimination of noise in the higher frequency ranges, above 300 Hertz, is somewhat easier than the reduction or elimination of resonant type noise which tends to be in the lower frequency range.
Resonant type noise in combustion driven heating apparatus has been dealt with in a variety of ways and to various degrees of success by others in the field. For instance U.S. Pat. No. 5,525,056 discloses a method of isolating the burner from the fuel supply by use of slots and acoustic baffles in the combustion chamber. These features are intended to prevent the burner from oscillating in phase with the combustion chamber. Likewise U.S. Pat. No. 4,090,558 discloses a method of treating resonance type noise at the burner itself by utilizing a separate heat exchanger having a circuitous flow path and passive resonators within the burner assembly to absorb objectionable noise. In yet another example, U.S. Pat. No. 5,435,716 treats the noise problem within the combustion chamber. In the '716 patent a method of absorbing acoustic energy is disclosed wherein a flexible membrane is installed in the combustion chamber wall. The membrane expands and contracts in response to pressure fluctuations within the combustion chamber, thereby absorbing acoustic energy.